Since physically active and physically fit children appear to have lower levels of some cardiovascular risk factors and since physical activity in childhood may predict adult health-related exercise, it is important to study physical activity in children. Current research is hampered by difficulties in measuring physical activity and fitness in children. The overall purpose of the proposed research is to provide needed data on the reliability and validity of promising measures of physical activity and physical fitness in children and adolescents. Two separate studies will be conducted. The aims of Study 1 are to assess the reliability and validity of the 7-day Physical Activity Recall (PAR) in children. The PAR has been validated for adults, and there is preliminary evidence of its validity in children. The test-retest reliability of the PAR will be studied in detail by collecting two recalls with overlapping days. Validity will be determined by comparing reported time at various activity intensities to Holter monitor heart rate records for the same day. Factors which may influence the reliability of the PAR will also be studied, notably body mass (kg/m2) and memory. In addition, the week-to-week variability of physical activity will be determined through repeated measures. The aims of Study 2 are to assess the reliability and validity of three commonly used submaximal fitness tests in children, and to determine whether the predictiveness of these tests can be improved by using ratings of perceived exertion (RPE). The three submaximal tests will be cycle ergometer, 3-minute step test, and 1600 meter run. Each test will be administered twice, with a two-week interval, to study reliability. Results from the submaximal tests will be correlated with VO2 max, as determined by maximal treadmill testing, as a measure of validity. RPE will be used in regression equations in an attempt to improve the predictiveness of submaximal tests in children, since this has been found in adults. Thirty subjects (15 boys, 15 girls) will be recruited from each of the 5th, 8th, and 11th grades of San Diego City Schools. Subjects will be randomly selected from the pool of volunteers so that a variety of ethnic groups will be represented. Thus, the relationship of age to the reliability and validity of the physical activity and physical fitness measures can be determined.